This invention relates in general to hydraulic braking systems for vehicles, and in particular to an apparatus and method for prefilling secondary circuits of a hydraulic control unit of an anti-lock brake system ("ABS").
Anti-lock braking systems for vehicles are well known hydraulic systems. Hydraulic fluid is carried by brake lines or circuits between a master cylinder and wheel brake assemblies. A hydraulic control unit or housing, containing valves and other components such as a pump, is located between the master cylinder and the wheel brake assemblies. Through a controller, the valves and other components selectively control pressure to the wheel brake assemblies to provide a desired braking response of the vehicle.
In one type of ABS, a normally open solenoid valve functions as an isolation or inlet valve in a main brake line between a master cylinder and a wheel brake assembly. A normally closed solenoid valve in the main brake line functions as a dump or outlet valve between the wheel brake assembly and an accumulator or expander which can receive fluid from the wheel brake assembly when the dump valve is opened. A pump is provided to pull fluid from the expander back to the master cylinder and the braking circuit. The portion of an ABS between the normally closed outlet valve and the pump is known as a secondary circuit. The balance of the hydraulic circuit of the ABS is known as the primary circuit.
During assembly of a vehicle, hydraulic brake fluid is added to the ABS. In most instances, the electrical connections between the ABS and its power source have not been made at the time when brake fluid is added. When brake fluid is introduced to the ABS, usually through a reservoir at the master cylinder, the volume of the secondary circuit does not receive fluid since the normally closed solenoid valve and pump block fluid from entering this section. Electrical power must be connected to the ABS later in the assembly process and the outlet valves actuated in order to actuate the normally closed solenoid valve and pump to fill the secondary circuit with brake fluid.
In a known ABS, lip seals are incorporated into the normally closed dump valves. During the evacuation process, the vacuum connected to the primary circuit pulls air past the lip seals to suck air from the secondary circuits. The lip seals acts as one-way check valves to permit air to pass from the secondary circuits while blocking brake fluid into the secondary circuits. During a dynamic vehicle test at the end of the assembly line, the secondary circuits are filled with brake fluid when the dump valves are opened.
In another known ABS, a vehicle assembly plant must manipulate ABS components in order to evacuate and fill the secondary circuits. Internal bleed screws are provided for each circuit and shipped to the assembly plant one-half turn from a seated position. Plungers for fluid accumulators must be pushed into a bypass position. Following the evacuation and fill process, the bleed screws must be tightened to seated positions and plunger tools must be removed.